1. Field of the Invention
The present invention relates generally to noise protection circuits, and more specifically to a circuit arrangement where noise-affected inbound signals received from a transmission medium are compared with a decision threshold to determine their logical level before processing the signals.
2. Description of the Related Art
Electromagnetic shielding of conductors is the usual technique for protecting integrated circuits from interference if the conductors are exposed to noisy environment. Care is also exercised to provide sufficient separations between such conductors and potential sources of noise. However, recent tendency toward high-density integration and layout and high transmission speed makes the current noise protection scheme ineffectual.
Particularly, in an integrated circuit where an inbound signal is received over a transmission medium, a noise is introduced to the received signal by electromagnetic coupling from an adjacent transmission medium when there is a transition in an outbound signal transmitted through it. As specifically shown in FIG. 1, such an integrated circuit includes a processing circuit 10 which receives input data from an input circuit 11 and forwards the result of processing as output data to an output circuit 12. The inbound signal of the integrated circuit is received over an input line 13 and supplied to the input circuit 11, which is essentially comprised of a comparator 14. Comparator 14 is enabled in response to a sampling pulse from the processor 10 to compare the received signal with a reference potential, or decision threshold that is supplied via an electromagnetically shielded conductor 15. Depending on the relative voltages of the input signals, the comparator 14 determines the voltage level of the inbound signal and produces an output as input data of the integrated circuit. This input data is supplied to the processing circuit and latched. The output data from the processor 10 is amplified through data buffers 16 and 17 of the output circuit 12 and forwarded onto an output line 18 as an outbound signal.
The input and output lines 13 and 18 are inductively coupled together by parasitic mutual inductance. As a result of this inductive coupling, a noise 20 is introduced to the inbound signal, as illustrated in FIG. 2, when the forwarded outbound signal changes its level. If the voltage transition of the outbound signal occurs when the inbound signal is low and its direction is upward as indicated by numeral 20 in FIG. 2, a voltage hump, or noise 21 would be introduced to the low-level inbound signal. If the noise level is higher than the reference potential and coincides with a sampling pulse 22, the comparator 14 detects the noise as a high-level input and produces a false output. Similarly, if the voltage transition of the outbound signal occurs when the inbound signal is high and its direction is downward as indicated by numeral 23 in FIG. 2, a voltage drop, or noise 24 would be introduced to the high- level inbound signal. If the high-level inbound signal drops to a level lower than the reference potential and coincides with a sampling pulse 25, the comparator 14 will detect this noise as a low-level input and produces a false output.
If the sampling pulse is out of timing with respect to the noise as indicated by a pulse 26, the comparator produces no false output. However, since the sampling pulses are not synchronized with the output data, the comparator 14 produces decision errors with a high likelihood of occurrence.